(a) Field of the Invention
This invention relates to a digitizer serving as an input device of digital unit such as computer and more particularly to an electromagnetic induction type digitizer using phase inversion detection system.
(b) Description of the Prior Art
Conventionally, this kind of digitizer is disclosed in U.S. Pat. Nos. 4,368,351 to Zimmer and 4,206,314 to Prugh et al.
Hereinafter, the conventional digitizer will be explained together with the drawings.
FIG. 4, FIG. 5 and FIG. 6 are conventional digitizers respectively and FIG. 7 is a timing diagram showing the operation waveform of each section.
FIG. 8 is an explanatory diagram showing the relationship between a signal induced on conductive lines and a position of a winding coil.
In FIG. 4, the reference numeral 1 is a tablet serving as a write-in plane plate for the position of the winding coil on which X-conductive lines and Y-conductive lines are arranged.
The conductive line X.sub.i is arranged in parallel to the conductive line Y.sub.j and the distance between the conductive line X.sub.i and conductive line Y.sub.j is 6.4 mm.
One end of the conductive lines X.sub.i and Y.sub.j is grounded commonly, and the other end of the conductive line X.sub.i is connected to the X-scanning circuit 102 and the other end of the conductive line X.sub.j is connected to the Y-scanning circuit 103.
The X-scanning circuit 102 and the Y-scanning circuit 103 are analogue switching means which are composed of such as analogue switch, and select one of the X-conductive lines and one of the Y-conductive lines respectively in response to a conductive line select address signal and further are connected to the input of the process circuit 110.
The reference numeral 4 is multi-stage binary counter which counts signals from an oscillator 5.
Certain bits of the count value functions as the conductive line select address signal a inputted to the X-scanning circuit 102 and the Y-scanning circuit 103.
In the construction of FIG. 4, bit signal a adjacent to the conductive line select address signal is used to change the X-scanning circuit 102 into the Y-scanning circuit 103 alternately, that is, the operation for selecting X-conductive line X.sub.i in turn and the operation for selecting Y-conductive line Y.sub.j are conducted alternately.
The following description will be made without distinguishing X-coordinate from Y-coordinate since the process for solving the value of X-coordinate is the same as that of Y-coordinate.
The bit signal a.sub.2 lower than the conductive line select address signal is provided to an energizing circuit 6 to energize the winding coil 7, and the signal process circuit 110 as a reference signal f in order to make phase detection in the induction signal e from the scanning circuit.
The winding coil 7 is energized by the energized signal b in response to this signal a.sub.2 and produces magnetic field so that the induction signal e is produced on the conductive line with electro-magnetic coupling.
Moreover, the lower bit a.sub.3 including the signal a.sub.2 of the count value of the count 4 is used to measure the position of the winding coil between the conductive lines and the detailed explanation of it will be described later.
The signal process circuit 110 is connected to the scanning circuit which receives the induction signal e induced on the conductive line.
The signal process circuit 110 includes an amplification circuit 11, a phase detector 12, a low pass filter 13 and a comparator 14.
Although the detailed explanation will be made later, a load signal i is outputted at a time when the position of the winding coil 7 above the tablet is detected.
The coordinate register 8 is loaded with the count signal j of the counter 4 in response to the load signal i so that the coordinate register 8 holds the count signal as the coordinate value. Next, the operation of the digitizer constructed as the above will be described with reference to FIGS. 5-8 and particularly by the timing diagram of FIG. 7.
The conductive line select address signal a is applied to the scanning circuit every certain period as shown in FIG. 7 and the energizing signal b of half period of the conductive line select address signal is applied to the winding coil 7 so that the induction signal e induced on each conductive line is inputted to the signal process circuit 110 in turn.
Compared the energizing signal b with the induction signal e, these signals are coincident of each other in the phase in the section between A and B and are in inversion phase to each other in the section between B and C.
As shown in FIG. 8, this is because the direction of the induction current is in the inverse state since the rotational direction of the magnetic field produced by the current I flowing into the winding coil, in the right side of the center O of the winding coil is opposite to that in the left side of the center O of the winding coil.
In FIG. 8, the center O of the winding coil is on one conductive line S.sub.K but the induction signal is not produced on the conductive line S.sub.K.
The induction signal e is inputted to the phase detector 12 after it is amplified to desired amplitude by the amplification circuit 11 of the signal process circuit.
The phase detector 12 has two input terminals, and it outputs a positive polarity signal when one input signal is the same polarity as the other, whereas it outputs a negative polarity signal when one input signal is opposite to the other in the polarity.
This circuit in U.S. Pat. No. 4,368,351 is embodied by the analogue multiplier of FIG. 5 and that in U.S. Pat. No. 4,206,314 is embodied by the sample hold circuit of FIG. 6.
The timing diagram of FIG. 7 corresponds to the analogue multiplier of FIG. 5 in which the induction signal e is multiplied by the reference signal f equal to the energized signal in view of phase.
The phase detection signal g is applied to the low pass filter connected to the low pass filter 13 which produces an envelope signal h forming an envelope line.
This digitizer adopts coordinate detection principal in which the induction signal induced on the conductive line at one side of the winding coil is opposite to that at the other side of the winding coil in view of phase.
In accordance with the principle, when the conductive line is selected in turn, the phase of the induction signal is watched and the conductive line on which the phase inversion occurs is detected, it may be found that the winding coil is disposed between the conductive line and one before the conductive line.
Moreover, this principle is used to detect not only a conductive line adjacent to the winding coil but also solve the detailed position of the winding coil between the conductive lines.
As shown in FIG. 7, the signals p, q and r in the neighbourhood of the conductive line on which the phase inversion occurs are brought amplitude change dependent on the position of the winding coil so that the polarity inversion point of the envelope signal h changes dependent on the amplitude change.
The time when the polarity of the envelope signal inverts represents the position of the winding coil.
The comparator 14 which receives the envelope signal h, produces the load signal i inverting its polarity at a time when the envelope signal passes "0".
From the above principle, the coordinate of the winding coil is solved by measuring time taken from a count starting point A (which is a scan starting point, coordinate "0" point) to a point B where the load signal i is produced.
Although the count value of the counter 4 is used to measure this time, the count value is used to count the conductive line.
In the example of FIG. 7, the space between two conductive lines is divided into four by the count signal j because it has 1/4 period of the conductive line select address signal.
This fact means that the coordinate is possible to solve every 1.6 mm, if the conductive lines are arranged in the distance of 6.4 mm between two conductive lines.
It is apparent that the count signal of shorter period is used in the case of solving a detailed position of the winding coil.
As mentioned above, this digitizer needs to extract the signal component holding the phase relations of the induction signal induced on the conduction lines, relative to the winding coil.
To realize this, the prior art technology has used the analogue multiplication and sample hold circuit as mentioned above.
However, these circuits are relatively complicated and expensive so that the circuits make it impossible for the digitizer to lower its cost.
Particularly, this digitizer using phase inversion detection system is possible to realize with the small size circuit construction and it has been desired to omit or simplify a phase detector since the signal process circuit makes it possible for amplification circuit, filter and comparator to be constructed with the common circuits.